12.4V DC open but drops to near 0 when load applied - Page 2 - SailNet Community
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post #11 of 21 Old 05-29-2017
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Re: 12.4V DC open but drops to near 0 when load applied

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Originally Posted by boatpoker View Post
Sounds more like a dead battery. If there was that high a resistance in the conductor, the fuse should blow (if there is one) or if it's a 20amp fuse on a 5amp pump or that last strand should melt.
Try putting the voltmeter probes directly on the lead posts of the battery terminals and then have someone flip the pump switch. If the battery voltage nosedives, you've found the problem.
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post #12 of 21 Old 05-29-2017 Thread Starter
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Re: 12.4V DC open but drops to near 0 when load applied

Wow, thanks for all the responses, everyone! I'm gonna go try some of these things now and report back.
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post #13 of 21 Old 05-29-2017
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Re: 12.4V DC open but drops to near 0 when load applied

Had this problem 2x now. First time it was an internally corroded wire (when cut open a large section had turned green and black almost like tar) which was a pain to find but after a "screw this I'll just start pulling wire out" moment I found it. Original wire on my boat, can't blame it for dying after nearly 40yrs.

This year however I blamed my brand new tiller pilot installation for the same issue. I thought I had a short or at least a bleed in it somewhere. Turned out both my deep cycle batteries finally kicked the bucket. Tell tale signs were there, batteries were "fully charged" as per my shore power charger, after 30mins would drop to 0-9v and everything would shut down, after only 5mins of being plugged into the charger they would be "full" again. 300$ to replace. Luckily not too bad on a small boat.

Anything else running funky other than the pump? Have you tried powering it up while plugged into shore power?

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post #14 of 21 Old 05-29-2017
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Re: 12.4V DC open but drops to near 0 when load applied

Is it just this pump, circuit or do you loose 12.4 V throughout the vessel?


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post #15 of 21 Old 05-29-2017
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Re: 12.4V DC open but drops to near 0 when load applied

Best was to isolate the ground is to pull a temporary new ground wire to the pump, If everything's hunky dory then its the ground path. If that doesnt work take a new Pos wire to it. Make sure its fused.
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post #16 of 21 Old 05-30-2017
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Re: 12.4V DC open but drops to near 0 when load applied

One more thing to check is the fuse. I have had a partly blown fuse which gave the same symptoms had me tearing my hair out for hours

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post #17 of 21 Old 05-30-2017
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Re: 12.4V DC open but drops to near 0 when load applied

If other substantial loads run OK (e.g. bilge pump, starter), it's probably not the battery itself. Bad connection(s) somewhere along the way most likely.
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post #18 of 21 Old 05-30-2017 Thread Starter
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Re: 12.4V DC open but drops to near 0 when load applied

Didn't get a chance to test it yesterday as I'd hoped, but just a little more info: I'm inclined to think it's an issue with the panel the pump and tillerpilot are connected to. I have 2 6-switch fuse panels. Circuits on one of them work fine (cabin lights, nav lights, depth, radio, accessory sockets). Other panel has the pump and tillerpilot, as well as anchor/strobe/tricolor and foredeck ligths. Had to leave before it was dark enough to see if those lights are working, which might confirm my panel wiring theory.

Boat was on the shorepower charger throughout all this testing, so I assume a battery problem -- even if there is one -- isn't the culprit. Hopefully will make it to the boat tonight.

Last edited by bajaking; 05-31-2017 at 01:35 AM.
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post #19 of 21 Old 05-30-2017
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Re: 12.4V DC open but drops to near 0 when load applied

If old syle fuses corrode emery cloth both the ends and replace in scraped cleaned fittings ,.Crud under screws need cleaning too. Think like an electron, if you can't get though, you don't..
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post #20 of 21 Old 05-31-2017 Thread Starter
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Re: 12.4V DC open but drops to near 0 when load applied

Quote:
Originally Posted by mitiempo View Post
Sounds like the supply wire - either positive or negative - has a bad connection, probably at the panel. It only takes one strand of wire connected for the voltage to pass, but it will not carry the required current for the pump to run.
Nailed it. Turns out the other circuits on this particular panel also didn't work; I mis-remembered that I've actually only use the other panel lately. Forgot to take my phone in the lazarette with me, but sure enough, the flimsy, unprotected automotive spade terminal attached to the flimsy, poorly soldered bus wire on the panel was corroded and virtually all but detached. It crumbled as soon as I touched it.

Message received, boat: you want your 42-year old electrical system updated.
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Last edited by bajaking; 05-31-2017 at 02:03 AM.
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